The present invention relates to horology, and, more particularly, to an electronic real-time clock having a low-power source with temperature compensation, real-time calibration, and error accumulation.
Electronic devices operating in a real-time environment often require accurate time-of-day clocks. A navigation or position-determining system which transmits accurately timed signals from a source or sources of known position such as earth-orbiting satellites to a user device having an indeterminate location, requires the use of an accurate time-of-day clock in the user device. An accurate clock in the user device is essential in such systems because an error therein relative to a system time reference standard can introduce unacceptable errors in the measurement of range from the transmitting source.
Oscillators which use a piezoelectric crystal for a frequency standard are known to be highly accurate but sensitive to changes in temperature. As the characteristic frequency of a piezoelectric crystal increases, e.g., to the mega-Hertz range, temperature stability increases; however, such high-frequency crystals are often large and not conducive to circuit miniaturization. It is further desirable in such systems that user devices which are movable, portable, or require storage for relatively long periods (months) consume minimal power. A user device having an extremely accurate, continuously powered time-reference generator, e.g., a crystal oscillator in a constant temperature vessel, as a sole time source may be impractical where portability and/or long term storage are desired. Moreover, electronic circuits controlled by high-frequency time sources operate at commensurably high switching speeds, thereby consuming excessive power in a portable device of the type, for example, having battery power. Present oscillator design constraints require that power consumption increase with increasing temperature stability; power consumption also increases with increasing pulse resolution. Accuracy and low power consumption are thus desirable but incongruous characteristics of portable electronic time pieces at the current state of the art.